Hollow fiber membrane modules for gas dehydration are known. See, for example, U.S. Pat. No. 5,002,590. Such modules employ a lumen-side feed and a countercurrent flow, and it has recently been discovered, operate more efficiently when a small portion of the dried product gas or raffinate is recycled to the outside or shell side of the hollow fiber membranes as a sweep gas. Because the pressure of the water vapor-containing gas is variable, and because there is always some pressure drop through the lumens of the fibers, it is difficult to design a module that permits a predetermined portion of the product gas to be returned as the sweep gas at a relatively constant rate. This need has been met by the present invention.